On Saturday, after decades of planning and years of headache-inducing construction, the $2.9 billion Metro Silver Line opens to passengers, linking Tysons and Reston to the D.C. region's extensive subway system. By 2018, the line will extend west another 11 miles, for an additional $2.7 billion, connecting to Dulles International Airport.

The long-awaited project is one of a series of large-scale construction jobs that have sought to ease congestion in a region known for its traffic jams.

The other projects, though, have been car-oriented: the eight-year, $676 million detangling of the Springfield Interchange, where Interstate 95 meets the Capital Beltway; $2.5 billion for the new Woodrow Wilson Bridge over the Potomac River connecting Maryland and Virginia, and the $2 billion Beltway Express lanes project, which added lanes for those willing to pay a traffic-variable toll.

As the Silver Line opening approaches, some still question whether the project is worthwhile.

Metro projects about 50,000 daily trips on the Silver Line one year from now on an average weekday. Nearly a third of those come simply because buses that currently deposit commuters to the Orange Line will be rerouted. Eventually, ridership projections increase, especially after the line reaches Dulles. By 2025, Metro projects about 115,000 daily Silver Line trips.

Meanwhile, the Dulles Toll Road, which serves the same corridor, carries more than 300,000 cars a day. It has been losing riders as tolls have risen more than fourfold in the last decade, largely because toll-road revenue has been tapped to pay for the Silver Line.

The project has long faced questions about whether the projected ridership is worth the costs. During the Bush administration, federal planners said that only the portion opening this week qualified for federal funds. (Eventually, the Obama administration did agree to provide low-interest loans for Phase II of the project.)

The FTA is now a fan of the project. Deputy Administrator Therese McMillan said in a statement that the Silver Line "will bring more world class transportation options to the entire Washington metropolitan region."

Rep. Gerry Connolly, D-Va., who worked for more than a decade at the local and federal levels push the project through, said he is confident the Silver Line will be viewed by future generations as a success.

He said ridership projections often badly underestimate demand, and that most of the grumbling about failing to qualify for federal funding came during the Bush administration, which he said was hostile to mass transit.

Broadly, it only makes sense to link the Dulles corridor to the rest of the region, he said.

"I see it as absolutely essential to the region's future," Connolly said.

The Silver Line has also faced other complaints. Parking lots are notably absent from most of the new stations, as planners hope to encourage a traditionally auto-dependent region to ditch their cars and adopt a walkable lifestyle.

The Silver Line is already spurring a complete redevelopment of sprawling Tysons. High-rise developments are under construction around the Metro stations, prompting a separate critique that the Silver Line is a boon for developers more than for commuters.

"The impetus behind the project was the developers, very clearly," said Sally Horn, president of the McLean Citizens Association, which represents many neighborhoods near the stations. While she said Fairfax County has generally done a good job ensuring that the development benefits residents, the lack of parking has left many McLean residents unsure if they will benefit from the line.

In addition, some Metro riders will actually see a decrease in service to accommodate the Silver Line. Metro estimates that 10 percent of existing riders see a decrease in service, with 37 percent getting increased service and 53 percent unaffected. The decreases occur because the Silver Line creates a bottleneck at a tunnel that also serves the Blue and Orange lines. Blue Line riders have been particularly vocal about their displeasure, even though Metro estimates that affected riders will only have to wait two minutes longer on average during rush hours for a train.

Overall, though, the Silver Line gives Metro an opportunity to improve service in northern Virginia, said Jim Hughes, Metro's director of intermodal planning. The Silver Line is particularly attractive to Metro because it diversifies the network. Since Tysons is a job and shopping hub — Tysons Corner Center is one of the largest shopping malls in the country and employs 10,000 itself, as an example — it keeps trains filled with revenue-generating passengers beyond the traditional 9-to-5, suburb-to-downtown commute, Hughes said.

Mall workers who currently rely on an extensive network of bus routes to get to work, though, were dubious about whether the rail line would improve their personal commute.

Farzi Abbassi of Alexandria said she would have preferred the billions invested in the Silver Line had gone instead to improving the bus network. For her, the Silver Line won't help ease a commute that sometimes takes two hours. The weak link in her commute is a neighborhood bus that runs infrequently, with no service on Sundays and late hours when she needs it.